[DrChiper]

Ok, I could find no way to fix this via calibre: at least I could not find it.
So this is what I did:

1. In calibre, I deleted all libraries but kept the files, and closed calibre.
2. In Windows, I renamed the top directory name of those still present libraries with according files, by adding some name, say "ORG".
3. I opened calibre and created all libraries anew with the original intended names: no more path-name prefixes in the library names! I closed calibre.
4. In Windows, I deleted the contents of the just created libraries, and copied the contents of the previously renamed "ORG" directories into them.
5. I started calibre and all libraries with their contents could now be found using the single library name.
6. For every library, I issued the Library Maintenance->Library check to make sure the libraries were healthy.
7. I closed calibre and removed the "ORG" directories.
8. Job done.

Summarizing,

• the import worked to init the new calibre installation with plugins, saved searches, etc. from my original calibre on the old computer;
• the import does not worked to define the libraries in the new calibre installation in the same manner as they were on the old computer (duplicates, additional path names);
• however, with some tweaking this can be corrected;
• also the export from calibre on the old computer does not seem to include file metadata.opf; perhaps not needed?

I'm not sure whether this behavior is due to my handling, or some oversight, or that this is erroneous behavior of calibre.
Anyone?